Overdrive Off light Blinking

I hade my truck scand at a local garage do see what the Blinking Overdrive light was and he says it was the speed sensor i changed it and it still blinks anyone no what else it might be or any way i can scan it my self i have a elmscan works good but the software dont scan the tranny abs or airbag

it is most definetely your tranny. mine did the same thing, but shifted fine. took it to the tranny place and found all kinds of internals had broke. the guys said the trucks do this before they really screw up the tranny. so it only cost me 400 instead of 2200 to fix my transmission.